Fluid catalytic cracking (FCC) is a well-known method for converting high boiling hydrocarbon feedstocks to lower boiling, more valuable products. In the FCC process, the high boiling feedstock is contacted with a fluidized bed of catalyst particles in the substantial absence of hydrogen at elevated temperatures. The cracking reaction typically occurs in the riser portion of the catalytic cracking reactor. Cracked products are separated from catalyst by means of cyclones and coked catalyst particles are steam-stripped and sent to a regenerator where coke is burned off the catalyst. The regenerated catalyst is then recycled to contact more high boiling feed at the beginning of the riser.
Typical FCC catalysts contain active crystalline aluminosilicates such as zeolites and active inorganic oxide components such as clays of the kaolin type dispersed within an inorganic metal oxide matrix formed from amorphous gels or sols which bind the components together on drying. It is desirable that the matrix be active, attrition resistant, selective with regard to the production of hydrocarbons without excessive coke make and not readily deactivated by metals. Current FCC catalysts may contain in excess of 40 wt. % zeolites. At these high zeolite concentrations, it is difficult to maintain a pore structure that is highly mesoporous while at the same time highly active and selective.
U.S. Pat. No. 5,221,648 describes a FCC catalyst which is a composite of a crystalline aluminosilicate zeolite within a mesoporous silica-alumina matrix. The matrix has a polymodal pore size distribution and is attrition-resistant and selective in the production of olefins. U.S. Pat. No. 4,908,405 relates to a FCC process employing a catalyst composition comprised of a monodispersed mesoporous aluminosilicate matrix material having pore diameter between about 100 and 500 Angstroms, alumina and a crystalline zeolite. U.S. Pat. No. 4,010,116 is directed to zeolite catalysts having improved thermal stability. The catalysts incorporate a synthetic mica-montmorillonite aluminosilicate, in admixture with a pseudoboehmite, AlO(OH)--H.sub.2 O. The pseudoboehmite may contain crystalline aluminum trihydroxides, Al(OH).sub.3, such as bayerite and gibbsite. Upon calcination at 500.degree. C., pseudoboehmite converts to gamma alumina. Therefore, a fresh catalyst containing pseudoboehmite would contain increasing amounts of gamma alumina as it ages in the FCC unit.
It would be desirable to use gibbsite in the inorganic matrix of a FCC catalyst because it is abundant and inexpensive. However, gibbsite is known to have a low surface area and is relatively inert in terms of its activity and has therefore been little used in FCC catalysts.